224 



ALTERNATORS. 



static voltmeter is connected through the revolving contact- 

 maker alternately to the terminals of the armature and the 

 non-inductive resistance in the field circuit by means of the 

 throw-over switch. The readings are repeated for a 

 succession of positions of the contact maker. The value 

 of the exciting current is calculated from the observed voltage 



Tfl 



E and known resistance E lf since = -^ 



1 

 Fig. 104 shows some results obtained in this manner 



with a small Pyke & Harris inductor alternator. The vertical 



2 



.Si 110 



100 



90 



40 



50 



O 1O 20 3O 



Divisions on Contact-Maker. 

 FIG. 104. CURVES SHOWING EFFECT OF ARMATURE CURRENT ON FIELD CURRENT. 



Excitation 1'6 R.M.S. amperes. 

 Armature current 1'5 K.M.S. amperes. 



scale is plotted in terms of the volts registered by the electro- 

 static wattmeter, the horizontal scale indicating divisions on 

 the contact-maker, which in this case were of an arbitrary 

 value. From the known resistance of the armature circuit 

 and that in the field circuit, the instantaneous values of 

 armature and field current inay be obtained. The values of 

 these resistances were 1 9 '3 ohms in the armature circuit, 59 ohms 



